I sold my Matrix, too. I felt that it was changing a the mids a little bit. The thing is my old, cheap PA power amp sounds better in a Slaving setup but has not enough power and is heavy, so I'm looking for a linear light weight power amp in the moment. I don't want a tube power amp because those that I tried coloured the sound too much.

It could work, but it would be much better if you get the Suhr Reactive load so you don't have to use the 1 x 12 cabinet as a load. I'm not familiar with the Weber mini mass 50 but some units have a low pass filter on the line out which would make your amp sound dark. Since a typical Marshall cab with Celestion only goes up to 5 or 6Khz it actually acts like a "lowpass filter" I use an RJM mini mixer and eq before going into the power amp. The eq gives you options to tailor your tone depending on how loud you're playing or what type of tone you want
You could also split the signal to ad effects if you have a mixer. I run one Marshall 1960B cabinet in stereo or 2